The Kaesong industrial park seen from a South Korean observation post (Picture: Reuters)

North Korea has barred South Korean workers from an economically and symbolically crucial industrial park, ratcheting up tensions in the region further still.

Hundreds of South Koreans were refused entry to the Kaesong factory zone, which accounts for almost $2billion a year (£1.3bn) in trade for North Korea and is located just north of the world’s most heavily-armed border.

It comes amid escalating hostility from Pyongyang, which has threatened to stage nuclear and missile strikes on Seoul and Washington and said the armistice ending the 1950s Korean War is void.

Kaesong was inaugurated in August 2000, containing 123 companies and employing 50,000 North Koreans and hundreds of South Korean business owners and managers.

The park is seen as a significant barometer of inter-Korean relations and one of the last remaining symbols of détente between the two countries.

Generally the two sides do not allow their citizens to travel to the other country without approval, but an exception has been made for South Koreans working at Kaesong, who are granted permission on a daily basis to cross into the park.

No entry: South Korean trucks return after they were barred from entering the Kaesong industrial park (Picture: AFP/Getty Images)

Kim Hyung-suk, a spokesman for Seoul’s Unification Ministry, said hundreds of his country’s employees were being allowed to return home but around 480 who planned to travel to the park to work today were refused entry.

He called for the ban to be lifted ‘immediately’ and said North Korea had cited recent political circumstances on the Korean peninsula when they delivered their decision to block entry to Kaesong.

There was concern among the hundreds of South Korean workers waiting to get in on Wednesday.

Lee Eun-haeng, who runs an apparel firm in Kaesong, said: ‘Trust between North and South will fall apart, as well as the trust we have with our buyers. We’re going to end up taking the damage from this.’

Pyongyang threatened to shut down the park last week but analysts had not expected it to jeopardise such a lucrative source of income.

Cheong Seong-chang of the Sejong Institute think tank in Seoul said: ‘It appears to be a temporary measure intended to raise tensions with the South.’

Questions: A South Korean man is surrounded by media after returning from Kaesong (Picture: AP)

North Korea’s threats to the US came after fresh UN sanctions were imposed for a third nuclear weapons test in February.

US defence secretary Chuck Hagel described the country’s development of nuclear weapons as a ‘growing threat’ to America and its allies.

On Tuesday secretary of state John Kerry told North Korea its recent rhetoric and actions were ‘provocative, dangerous and reckless’ and warned: ‘The United States will not accept the DPRK as a nuclear state.’

He told reporters during a joint news conference with South Korean foreign minister Yun Byung-se that North Korea knows the US is fully prepared and capable of defending itself and its allies.